Field of the Invention and Related Art Statement
The present invention relates to an apparatus for detecting a moving direction of a light beam spot illuminated on an optical information recording medium having tracks formed thereon, such as an optical card and an optical disc.
In an optical information recording and/or reproducing device, optical information is written and reproduced by illuminating a light beam spot on the track formed on the optical information recording medium. In order to correctly write and read the optical information on the optical information recording medium, it is necessary to scan the track by the light beam spot accurately. For this purpose, in the known apparatus, a deviation amount of the light beam spot from the track is detected to obtain a tracking error signal and the position of the light beam spot on the recording medium is adjusted in accordance with the deviation amount of the light beam spot. That is to say, a tracking servo operation is effected in accordance with the tracking error signal.
However, in the known apparatus, the tracking servo operation is effected by processing only the tracking error signal. Therefore, if some external force is applied to the optical information recording medium, the light beam spot could not follow the track accurately. Further, in a seek operation mode in which the light beam spot is moved across successive tracks at a high speed into a desired track, a long time is required to obtain a stable tracking servo operation.
In a known information recording and/or reproducing apparatus disclosed in Japanese Patent Laid-open Publication No. 63-179476, the above problem is solved such that the moving speed and direction of the light beam spot moving in a tracking direction perpendicular to the direction of the track are detected and the tracking servo operation is started promptly in accordance with the thus detected moving speed and direction of the light beam spot.
FIG. 1 is a block diagram showing the construction of the known apparatus for detecting a moving direction of a light beam spot disclosed in the above Japanese Patent Laid-open Publication No. 63-179476, and FIGS. 2A to 2I are signal waveforms appearing at various points in the apparatus shown in FIG. 1. In this known apparatus, three light beams are projected on the optical information recording medium, a main light beam among said three light beams being used to write or read the information and the remaining two sub light beams being projected for detecting the tracking error. In order to detect the tracking error, there are provided two photo detectors 1 and 2 for detecting intensities of said two sub light beams reflected from the optical information recording medium. The outputs from the photo detectors 1 and 2 are supplied to an adding circuit 3 and a subtracting circuit 4 to obtain a deviation amount signal TAB (FIG. 2A) from the adding circuit 3 and to obtain a tracking error signal TE (FIG. 2B) from the subtracting circuit 4, respectively. The signal TAB represents a deviation amount of the light beam spot from the track and the signal TE a deviation amount and direction of the light beam spot from the track. The signal TAB is further supplied to a first comparator 5 and the signal TE to a second comparator 6, respectively. In these comparators 5 and 6, the deviation amount signal TAB and the tracking error signal TE are compared with predetermined values to produce a bivalent signal TL and a bivalent signal TZ, respectively as shown in FIGS. 2C to 2D. The bivalent signals TL and TZ are supplied to D input terminal and clock terminal CK of a D flip-flop 7, respectively. In the D flip-flop 7, the signal TL is synchronized with the leading edge of the signal TZ to produce a moving direction signal FWD/REV from an output terminal Q of the D flip-flop 7. It should be noted that when the light beam spot moves in the forward direction the moving direction signal becomes low, and when the light beam spot moves in the reverse direction the moving direction signal becomes high.
However, the known apparatus has a problem as will be explained below.
On the track formed on the optical information recording medium, a prepit such as sector information is preliminarily recorded. When the light beam spot passes over the prepit, the level of signal TL becomes temporarily low as shown in FIG. 2H by the cross-hatched area. If the undesired low level signal is generated in synchronism with the leading edge of the signal TZ (FIG. 2G), the moving direction signal FWD/REV is inverted from a high level to a low level as shown in FIG. 2I, although the actual moving direction of the light beam spot is not reversed. While the level of the moving direction signal FWD/REV is kept low, the servo system operates in as the positive feed back mode and thus the relative moving speed between the light beam spot and the track is accelerated. Therefore, a problem results in that a long time is required to pull back the tracking servo operation into a stable condition. It should be noted that if a dust particle is present on the information recording medium or a flaw is formed therein, the same problem would occur.